The present invention generally relates to a radiation tomography apparatus for performing tomography of an object under inspection utilizing radiation, and for performing checking/measurement of the object based on the obtained tomographic image.
A computer tomography scanner (hereinafter referred to as a CT apparatus) is known as a typical radiation tomography apparatus. The CT apparatus can non-destructively check for internal defects, composition, structure, etc. of objects, and can perform precise measurement.
The CT apparatus normally has a radiation source and a radiation detector. The radiation source radiates a fan beam X-ray which diverges in a sector plane. The detector is opposite to the radiation source with the object interposed therebetween. The detector has a plurality of radiation sensor elements arranged along the diverging direction of the fan beam X-ray. The radiation source and detector, having the object interposed therebetween, are rotated in one direction in units of degrees within a range of 180 to 360 degrees. Radiation scanning is thus performed. After obtaining X-ray absorption data of an object slice from many directions, processing means, such as a computer, preforms an image reconstruction operation. A tomographic image is then obtained.
In the CT apparatus, for each object slice, an image can be reconstructed with the order of 4,000 gradation steps in accordance with the object composition. From this, the state or condition of the object slice can be inspected in detail.
A CT apparatus as described above is of a so-called third generation CT apparatus. First, second and fourth generation CT apparatuses are also known as other types of the CT apparatus.
A first generation CT apparatus has an X-ray source for generating a pencil beam X-ray and a detector opposed to it. The X-ray source and detector conduct traversal scanning (parallel and straight scanning of X-ray beam) along the slice of an object. Each time one traverse scan is completed, the X-ray source and detector are rotated by a predetermined angle. Thereafter, another traverse scan is similarly performed.
A second generation CT apparatus is an improvement over the first generation CT apparatus. A narrow fan beam X-ray is used in place of the pencil beam X-ray. The detector has only a small number of sensor elements. The X-ray source and detector perform traverse scanning with rotation.
A fourth generation CT apparatus has a detector with sensor elements arranged along the entire circumference of an object, and has an X-ray source for radiating a wide-angle fan beam X-ray. In this apparatus, only the X-ray source is rotated.
In industrial fields, internal defects of products have often be be checked non-destructively. In view of this, the use of a CT apparatus for this purpose has recently been considered promising. FIGS. 1A, 1B and 1C respectively show schematic structures of CT apparatuses of the first, second and third generations. Referring to these figures, reference numeral 1 denotes a radiation source using an X-ray tube or a radioisotope (hereinafter referred to as RI); and 2 denotes a detector. An object to be inspected is placed on table 3 which may be a simple one. Table 3 can be rotated or parallel-shifted so as to allow easy testing and measurement of even large objects.
The first generation CT apparatus in FIG. 1A has radiation source 1 and detector 2 which are fixed opposite each other. Source 1 emits pencil beam radiation B1. Detector 2 has a single sensor element for detecting radiation B1. Table 3 is interposed between source 1 and detector 2. An object (not shown) mounted on table 3 is traversely-scanned in the directions indicated by arrow A. Table 3 is rotated in the direction of arrow B and data is obtained upon completion of each traverse scan.
The second generation CT apparatus shown in FIG. 1B has basically the same construction as FIG. 1A. However, source 1 radiates fan beam radiation B2 in place of pencil beam radiation B1. Detector 2 has a plurality of sensor elements corresponding in number to the spread width of radiation beam B2.
In the third generation CT apparatus shown in FIG. 1C, source 1 generates fan beam radiation B3 having a given spread width which is capable of covering the entire area of table 3. Detector 2 has a plurality of sensor elements corresponding in number to the spread width of radiation B3. Source 1 and detector 2 are fixed and opposite each other with table 3 interposed therebetween. Table 3 is rotated to acquire image data.
Such industrial CT apparatuses conventionally employ X-ray tubes for radiation sources. However, when products of materials having high radiation absorption coefficients or large-sized products are to be checked with X-ray tubes, accurate data being free of noises can be hardly obtained. In such a case, an RI with a high output energy is used.
Assume an object, such as a tire, that contains substances which have both high and low X-ray absorption coefficients, e.g., steel and rubber. If the object is irradiated with X-ray energy suitable for the lower X-ray absorption coefficient substance, the degree of X-ray absorption by the high absorption coefficient substance is too large. The resultant difference between the X-ray absorption of the high coefficient substance and that of the low coefficient substance causes a prominent artifact from the high coefficient substance. This artifact disturbs observation of an image of specific portions having the lower absorption coefficient.
To prevent such artifact, an object may be irradiated with high energy which is well fitted to the higher absorption coefficient. In this case, however, the degree of X-ray attenuation through the substance having the lower X-ray absorption coefficient becomes almost zero. Accordingly, image information of the low absorption coefficient substance is difficult to obtain.
The above problem also applies to a case wherein an RI is used as the radiation source. Theoretically, it is possible to deal with this problem in such a manner that data for steel (high X-ray absorption coefficient) is searched for and extracted from projection data arranged in a sinogram. Then, given interpolated data is put into the specific location of the sinogram, from which the searched data has been extracted. However, it is very difficult to detect the data for steel in a sinogram and, therefore, this manner is hard to actually reduced to practice.
Thus, there is no practically good manner to achieve a precise inspection of the composition of a substance which contains portions (e.g., steel portions and rubber portions) having largely different radiation absorption coefficients.